Digital X-ray tomosynthesis is an imaging technique that enables three-dimensional imaging of a patient using a large-area digital detector typically used for conventional (single projection) radiography. A finite number of projection images over a limited angular range, typically between 20° and 40°, are acquired by varying the orientations of the x-ray tube, patient and detector. This is usually accomplished by either moving both the detector and x-ray source or by fixing the position of the detector (source) and moving the x-ray source (detector). In applications where the detector is fixed, multiple spatially distributed X-ray sources may alternately be used, or movable sources may be displaced in various patterns or trajectories. Three-dimensional data is reconstructed from the captured projections in the form of a number of slices through the patient anatomy, each parallel to the detector plane. A consequence of limited angular scanning is that the in depth resolution is much lower than the in-plane resolution of the reconstructed object.
Reconstruction of volumetric data from a tomosynthesis system requires knowledge of the underlying capture geometry, including the relative orientation and position of the detector, the movement and position of the source for each projection, and potential patient motion. In a standard tomosynthesis apparatus, many of the geometric variables are well known, as the detector position is precisely specified, and the relationship between source and detector is mechanically fixed and well established.
For a bed-side portable tomosynthesis system, however, the capture geometry can be difficult to determine with the desired accuracy. Detector positioning is done by the operator with consideration for affording the patient a reasonable degree of comfort, but without a high degree of geometric precision. The detector is placed behind a patient by an attending operator, so that often the detector is completely obscured by the patient's body. For instance, the patient may be in a propped position, with the detector placed behind the patient. The angle between the detector plane and a horizontal plane is only approximately known. Moreover, the detector might be skewed with respect to the transport path of the x-ray source, which further complicates the reconstruction process. The result is that the image quality of the resulting volumetric data can be compromised.
There is a need for a calibration utility that overcomes the limitations aforementioned and more accurately defines the geometry of the tomosynthesis system when using mobile radiography apparatus.